Jumping

Both of them are arthouse shorts that revolve around exploring their world through the camera and both of them have somewhat similar animation style (not artstyle). 

At face value, these seem very different - one is a full series while the other is only six minutes long. However, the jist is the same - the characters' imagination is drawn out in front of them and they build the worlds in which they adventure themselves. In Jumping, you don't actually see the MC on screen, as it is shot in first person. Both animes are also directed by anime legends - Jumping's Osamu Tezuka and Eizouken's Masaaki Yuasa. Both animes are also incredibly beautiful and are all about setting.  

A short length and plot deficiency aren't the only things that put Tobira o Akete and Jumping together. Jumping into an urban landscape and flying through a variegated dream world gives you the exact same feelings, and definitely brings the two anime together.